1. Field of the Invention
The invention relates generally to substrate surfaces modified with vapor plasma deposited titanium nanoparticulates, which under certain deposition conditions can form spinulous surfaces. The nanostructured coatings are effective scaffolds for cell adhesion.
2. Description of Background Art
Metals have long been recognized as capable of growing tiny hairs, often called whiskers, under certain conditions. The mechanism causing such growth is not well understood, but is frequently observed under conditions of mechanical or thermal stress as well as under conditions peculiar to electroplating processes. In digital circuits at high frequencies, such whiskers are able to act like mini antennae, causing circuit impedance. It is well documented that whiskers are found in zinc, silver, tin, zirconium and gold filaments used in a variety of applications. Whiskers have been observed to grow from metal films and coatings over a period of time.
Whiskers are generally found as hair-like nanostructure projections with relatively high aspect ratios. While cross sections are generally polygonal, some cross sections are reported to vary as hexagonal, diamond or circular projections (US patent application publication No. 2007/0018139). Thermal evaporation of tin powders on titanium nitride coated substrates can result in tin oxide fishbone-like nanostructures (Kim, et al., 2005).
Whiskers can be deliberately “grown” via several processes; including manipulation of temperature and/or pressure. TiN whiskers can be synthesized by reacting ammonium chloride, titanium, titania in the presence of a meta/carbon catalyst under nitrogen at appropriately high temperatures, as described in U.S. Pat. No. 5,665,326. Wokulski (2001) employed a chemical vapor deposition process to prepare TiN whiskers from a titanium chloride, nitrogen and hydrogen gas mixture. Similarly there are many other reports of synthesis of compound whiskers; for example, transition metal carbides (U.S. Pat. No. 4,756,971).
Depending on the metal, whisker growth may occur at room temperature for low melting metals such as tin. Other metals such as aluminum (Al) and titanium (Ti) generally exhibit whiskers at much higher temperatures. Suzuki, et al. (2006), for example, studied the growth of Al whiskers from pure Al after deposition on a surface oxidized silicon substrate in an electron beam evaporation apparatus at high temperature. Russell, et al. (1958) reported growth of titanium whiskers from titanium wire thermally cycled in vacuum from about 800-1100° C.
It is possible that metal crystal growth and subsequent whisker formation is promoted by various impurities or other elements in the metal.
Compositions employing titanium whiskers as strengthening materials in thermoplastic compositions for orthopedic applications, such as interjoint and disc gap filling have been described in U.S. Pat. Application No. 2004/0228898.
Several studies over the past several years have focused on the growth and nature of nanostructured thin films, with a special interest in control of physical vapor deposition. Metal oxides deposited at glancing angles result in controllable columnar microstructures depending on substrate motion variation (Robbie and Brett 1997). The glancing angle technique (GLAD), produces vapor deposited thin film microstructures with distinct helical columnar appearance (U.S. Pat. No. 6,248,422; U.S. Pat. No. 6,206,065).
Nanostructured surfaces of GLAD films have been suggested as having possible applications in chiral optics and, due to magnetic anisotropy, in development of information storage devices because of the ability to deposit materials such as silicon in the form of nanostructured helical columns. Hawkeye and Brett (2007) reviewed GLAD films and foresee applications in solar energy conversion, fuel cells, gas sensors, catalysts and electrochemical capacitors.
GLAD films produced from electron beam heated silicon deposited on glass were studied McIntosh, et al. (2003) to assess hTert fibroblast morphology and survival on the columnar surfaces created from silicon deposited over a range of angles. Adhesion, spreading and survival beyond one day were observed only on surfaces deposited at a 70° angle, despite the identical composition of the columns deposited at other angles.